An image sensing element such as a CCD sensor, CMOS sensor, or the like has defective pixels called white spots with abnormally large dark currents, which often deteriorate the image performance of the image sensing element. By correcting such defective pixels by defect correction, the yield of expensive image sensing elements can be improved.
However, a high-end digital camera must provide high-quality images even under severe image sensing conditions such as high ISO (International Organization for Standardization) speed rating, long shutter time, and high temperature. The ISO speed indicates the sensitivity of a film to light, and is indicated by a converted value in a digital camera.
For this purpose, defective pixels under such worst conditions (i.e., high ISO speed, long shutter time, and high temperature) may be picked up, and undergo defect correction. However, in such case, information of pixels which do not have so large dark currents and can be used without correction is discarded, thus narrowing down an effective dynamic range.
In Japanese Patent Laid-Open No. 6-303531, white spots having temperature dependence are detected using a predetermined threshold value, and undergo defect correction. As a result, variations (dark current nonuniformity) equal to or smaller than the threshold value remain uncorrected in an image.
Furthermore, in Japanese Patent Laid-Open No. 11-239298, white spots (white scratches) which increase/decrease in accordance with the exposure time merely undergo defect correction in accordance with the exposure time, and nonuniformity of pixels which remain uncorrected cannot be removed.
In case of a system such as a digital still camera which changes the amplifier gain in correspondence with the ISO speed, the aforementioned problem is serious. In such system, an appropriate exposure value changes, and an output signal in an image changes. As for the output signal, amplification of the signal is controlled based on, e.g., the amplifier gain in consideration of the amplitude of A/D conversion.
For example, when ISO speed=100, a signal obtained from a light amount of 0.1 lux·sec is output as a 1× signal; when ISO speed=200, a signal obtained from a light amount of 0.05 lux·sec is output as a 2× signal; and when ISO speed=400, a signal obtained from a light amount of 0.025 lux·sec is output as a 4× signal.
That is, since the occupation ratio of dark voltage nonuniformity in the value obtained by an A/D converter changes upon a change in ISO speed (amplifier gain), a serious problem is posed. Note that the signal may be amplified not by the amplifier gain but by output control of an A/D converter or the like. However, in case of the output control of an A/D converter, it may become harder to control the signal than the amplifier gain.
Even when the A/D converter has bit precision or broad dynamic range with which the occupation ratio does not suffer, pixels originally having dark currents like white spots (white scratches) cannot be covered by only dark correction since they suffer large fluctuation components called dark current shot noise.